/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode* curr = nullptr, *pre = head;
        while(pre != nullptr) {
            ListNode* temp = pre -> next;
            pre->next = curr;
            curr = pre;
            pre = temp;

        }
        return curr;
    }
};
